making maps of things + non things - visual system

Question 1

what is the topographic map?

Answer 1

a two dimensional representation of the spacial position of stimuli on a sensory surface

Question 2

The topographic map in the visual system:

Answer 2

map of retinal ganglion cells in the retina
- now projected into tectum or superior colliculus

Question 3

How is the visual topographic map created?

Answer 3

Through gradients of Ephs and Ephrins
(Ephs are the receptors)

Question 4

What did the stripe assay demonstrate?

Answer 4

Temporal RGC axons demonstrate a clear preference for the anterior tectum
(nasal showed no preference between anterior and posterior)

Question 5

Why do temporal axons show a preference for the anterior?

Answer 5

The posterior tectum makes a non-permissive factor that repels the temporal axons

Question 6

What is the inhibitory factor expresses in the posterior tectum?

Answer 6

Eph A2 and A5

Question 7

What is the ephrin gradient?

Answer 7

High in posterior, low in anterior

Question 8

What is the ephrin gradient in the retina?

Answer 8

counter gradient
high in temporal, low in nasal

Question 9

KO of Ephrin A2 and A5

Answer 9

temporal axons project axons to the posterior tectum
- the topographic map is disordered

Question 10

How does synaptic competition play a role in formation of the mammalian visual topographic map?

Answer 10

1. initial growth occurs all over the tectum
2. axon branching + synaptogenesis occur
   - sensitive to the ephrin gradient
3. competition between axons for synaptic partners - involves electrical activity

weaker synapses get eliminated
epsins have an effect on strength of synapses

Question 11

How can electrical stimulation modulate responses to guidance cues?

Answer 11

low conc netrin = no response
low conc netrin + electrical stim = attractive response
- electrical stimulation heightens sensitivity

MAG + electrical stim = attractive response

Question 12

How does electrical activity modulate response to guidance cues?

Answer 12

Directly affects cAMP/cGMP balance
electrical stimulation increases cAMP
- is elevated in a calcium-dependent manner

Question 13

How does mapping take place before birth (no light)?

Answer 13

the retina becomes spontaneously active as axons reach the rectum
- activity is organised

waves of calcium pulse through - cells nearby fire together
= synapses are strengthened + they wire together

map refinement can be blocked with a sodium blocker

Question 14

What is the main RGD receptor in the postnatal period?

Answer 14

nACHR
mice w/out the beta2 subunit of nAChR = uncorrelated RGC activity (disrupted retinal waves + no refinement of topographic map)

Question 15

What neurotrophin is involved in electrical activity in the retina?

Answer 15

BDNF

Question 16

What does BDNF do?

Answer 16

promotes retinal ganglion cell axon branching
this only occurs where eph-ephrin interactions are low

Question 17

How do ephrins and Trks interact to result in different outcomes?

Answer 17

Ephrins and TRKB (receptors for BDNF) interact physically

1. BDNF binding to TrkB = increase in PI-3 kinase branching
2. BNDF binding to TrkB in the presence of low Ephrin = large increase in PI3K
3. BDNF binding to TrkB in the presence of high ephrin = decrease in PI3K and branching

Question 18

Where else are the ephrin gradients used?

Answer 18

in the visual system …
RGCs also project to the lateral geniculate nucleus
lGN receives inputs from both eyes
- allows stereoscopic vision